Mine would be [rec] It's a Spanish language zombie/supernational movie that made me actually scared, and I am a HUGE horror buff.
A favorite's too hard. I was a huge horror buff as a kid, then completely turned off by it, and a friend has me back to watching it quite a bit lately. As a kid, IT scared me the most - I'm still not crazy about clowns, but I was scared of them when I was little - the movie didn't help. lol I really love the simplicity of the original Halloween. And an old one I really enjoy is the 1925 Phantom of the Opera. Even though none make my top three, I've always had a soft spot for vampire flicks.
Its not really a horror movie.. but Deliverance freaked me out. Maybe it had to do with the fact that someone tried to break in my house while I was watching it? Well.. that and the fact I live on the Ohio River (in Ohio) and Kentucky and West Virginia REALLY DO have people like that.